FIG. 1 is a diagram for explaining an example of a conventional power supply voltage control method. In FIG. 1, a LSI (Large Scale Integrated circuit) 1 includes a power line 2 that has a resistance, an inductance and a capacitance, and transistors 3. A DC-DC converter 5 that converts a DC (Direct Current) input voltage into a predetermined DC output voltage supplies a converted power supply voltage VDD to the LSI 1 via a power line 6 that includes a resistance, an inductance and a capacitance. In FIG. 1, GND denotes the ground. In this example, a voltage at a power supply input terminal of the LSI 1, that is, an analog voltage signal, is fed back to the DC-DC converter 5 in order to control the power supply voltage VDD.
In the example illustrated in FIG. 1, the voltage at the power supply input terminal of the LSI 1 is monitored and fed back to the DC-DC converter 5. Because the voltage drop on the power line 2 does not necessarily stabilize, the power supply voltage that is actually applied to the transistors 3 within the LSI 1 may be unknown. Consequently, it is difficult to control the power supply voltage that is applied to the transistors 3 within the LSI 1 to a constant voltage.
It is conceivable to provide a dedicated line exclusively for monitoring the power supply voltage that is actually applied to the transistors 3 within the LSI 1. But in this case, a fluctuation in the power supply voltage at a high frequency may not be monitored unless the resistance of the dedicated line is reduced to a sufficiently small resistance. In addition, when the dedicated line is provided, it is necessary to separately provide on a package of the LSI 1 a terminal (that is, a signal pin) exclusively for the voltage monitoring. Furthermore, when a voltage monitoring point is provided at a central part within the LSI 1, it is necessary to route the signal lines within the package so as not to interfere with existing lines. Consequently, the degree of freedom of layout within the package of the LSI 1 deteriorates, and the area occupied by the lines increases to thereby limit the area in which circuit elements such as the transistors 3 may be provided within the LSI 1.
The applicant is aware of Japanese Laid-Open Patent Publications No. 2004-228104 and No. 2008-72045.
According to the conventional power supply voltage control method, there is a problem in that it is difficult to maintain the power supply voltage constant within the LSI using a relatively simple structure, without increasing the number of terminals of the LSI package.